The invention relates to a novel rhodium sulphide catalyst for reduction of oxygen in industrial electrolysers. The catalyst is highly resistant towards corrosion and poisoning by organic species, thus resulting particularly suitable for use in aqueous hydrochloric acid electrolysis, also when technical grade acid containing organic contaminants is employed.
The invention also relates to a process for the electrolysis of contaminated hydrochloric acid.
Hydrochloric acid is obtained as a waste product in a number of chemical processes. This applies in particular to addition reactions using phosgene, such as in isocyanate chemistry, where the chlorine used issues completely in the form of HCl. Hydrochloric acid is however also formed in substitution reactions, such as for example in the production of chlorobenzenes and chlorotoluenes, in which half of the chlorine used issues in the form of HCl. The third main source of HCl is the thermal decomposition of chlorine-containing compounds, in which chlorine issues completely in the form of HCl. If no direct use exists for the gaseous HCl, such as for example in oxychlorination processes, concentrated hydrochloric acid is formed by absorption in water or dilute hydrochloric acid. Chemically non-usable quantities can be very advantageously recycled to form chlorine by means of hydrochloric acid electrolysis, and in particular by means of hydrochloric acid electrolysis using oxygen-depolarised cathodes.
The electrolysis of aqueous HCl solutions is a well known method for the recovery of high-value chlorine gas. Aqueous hydrochloric acid is an abundant chemical by-product, especially in chemical plants making use of chlorine as a reactant: in this case, the chlorine evolved at the anodic compartment of the electrolyser can be recycled as a feedstock to the chemical plant. Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this context is crucially dependent on the nature and performance of the catalyst, but also on the structure of the gas diffusion electrode.
Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions; the activation of gas diffusion electrodes with platinum based catalysts is well known in the art, and finds widespread application in fuel cells and electrolysers of many kinds. However, the case of aqueous HCl electrolysis poses some serious drawbacks to the use of platinum as cathodic catalyst, as it is inevitable for the gas diffusion cathode to come at least partially in contact with the liquid electrolyte, which contains chloride ion and dissolved chlorine. First of all, platinum is susceptible to chloride ion poisoning which negatively affects its activity toward oxygen reduction; a second source of poisoning is constituted by contaminant species, especially organic species, which are in most of the cases dissolved in the by-product hydrochloric acid undergoing electrolysis. Even more importantly, the combined complexing action of hydrochloric acid and dissolved chlorine gas changes the platinum metal into a soluble salt which is dissolved away, making this material inappropriate for use in gas diffusion electrodes.
Other platinum group metals appear to follow a similar fate. For example, according to Pourbaix"" Atlas of Electrochemical Equilibria in Aqueous Solutions, finely divided rhodium metal dissolves in hot concentrated sulphuric acid, aqua regia, and oxygenated hydrochloric acid. Similarly, (hydrated) Rh2O3.5H2O dissolves readily in HCl and other acids. These problems have been partially mitigated with the disclosure of the rhodium/rhodium oxide based catalyst described in concurrent U.S. patent application Ser. No. 09/013,080. In particular, the rhodium/rhodium oxide system, although slightly less active than platinum towards oxygen reduction, is not poisoned by chloride ions. Also the chemical resistance to aqueous hydrochloric acid with small amounts of dissolved chlorine is sensibly enhanced with respect to platinum. However, an activation step is needed to obtain a sufficiently active and stable form of this catalyst, and some limitations arise when such catalyst has to be included in a gas diffusion electrode; for instance, the chemical and electronic state of the catalyst is changed upon sintering in air, a very common step in gas diffusion electrode preparations known in the art. Cumbersome and/or costly operations have to be carried out to replace this step, or to restore the active and stable form of the catalyst afterwards, as disclosed in U.S. Pat. No. 5,598,197. Furthermore, the required chemical stability is displayed only in the potential range typical of the electrolysis operation; extremely careful precautions have to be taken during the periodical shut-downs of the electrolysers, otherwise the sudden shift in the cathodic potential, combined to the highly aggressive chemical environment, causes the dissolution of a significant amount of catalyst, and the partial deactivation of the remaining portion. While tailored procedures for planned shut-downs of the electrolysers can be set up, although resulting in additional costs, little or nothing can be done in case a sudden, uncontrolled shut-down due to unpredictable causes (for instance, power shortages in the electric network) should occur. There is also no evidence that rhodium/rhodium oxide based catalysts are more insensitive to contaminants with respect to platinum based catalysts.
Technical-grade hydrochloric acid of the kind obtained for example in the above mentioned processes, is usually contaminated with partially chlorinated organic substances, such as for example monochlorobenzene or ortho-dichlorobenzene from the processes themselves, as well as possibly with organic substances from vessel linings, packing materials or pipelines. Such organic substances are obtained for example in the form of surfactants or acrylic esters. The total concentration measured in the form of the TOC can in fact greatly exceed 20 ppm. In the electrolysis of hydrochloric acid using oxygen-depolarised cathodes in initial tests in which platinum was used as the catalyst, the operating voltages were found to be sensitive to the degree of contamination: over a period of several weeks, and in some cases only a few days, an increase in the cell voltage by 150 to 300 mV was observed, a phenomenon which was at least partially reversed during experimental operation using chemically pure hydrochloric acid. Similar results were obtained after switching off the apparatus although the reduction in voltage did however disappear again after a few days. The object was to find a process which avoids this disadvantage of increased operational voltage in the presence of contaminated hydrochloric acid.
The hydrochloric acid typically recycled in production processes usually emerges from several feed streams with corresponding fluctuations in the content of organic or inorganic impurities. Besides the mentioned organic impurities typical inorganic contaminants are in particular sulphates, phosphates and sulphides. One attempt to solve this problem was the purification of technical grade hydrochloric acid using activated carbon. The effect of the reduction in the highly fluctuating TOC from between 20 and 50 ppm to approx. 10 ppm, accompanied by the reduction in the content of chlorinated organic substances to  less than 1 ppm, already produced a considerable improvement in the operation of the cell.
Subsequent purification of the concentrated, approx. 30% hydrochloric acid, with the aid of adsorber resins, allowed a reduction in the content of chlorinated organic substances to below the detection limit of 6 ppb. It was however also found that the non-chlorinated organic substances, which did after all make up the main proportion of impurities, rapidly exhaust the adsorptive capacity of the adsorber resin at the high impurity contents, so that these organic substances break through the adsorption column and have a negative effect on the operating voltage of the electrolysis. The cell voltage increases accordingly. The regeneration of the adsorber resin with methanol according to the manufacturers"" specifications would be relatively laborious and, given the above contents of impurities, would have to be carried out every few days. Due to the risk of explosion which must be taken into account the adsorber resin container would have to be removed and regenerated externally.
If the hydrochloric acid does however stem from a direct connection to an isocyanate unit the content of impurities is considerably lower and consists essentially of the constituents mono- and dichlorobenzene, which can be removed very successfully by means of activated carbon as well as adsorber resins to levels below the detection limit, and the regeneration cycles of the adsorber resin packing extend to several months up to about half a year, depending on the content of impurities.
Tests with platinum catalysed oxygen-depolarised cathodes all showed a similar high sensitivity towards organic impurities. In tests using rhodium oxide-catalysed oxygen-depolarised cathodes the sensitivity towards organic substances was found to be slightly less, although it was still quite considerable. The rhodium oxide catalyst had been developed in order to be able to dispense with polarisation upon switching the apparatus off. This catalyst did however reveal in tests that its structural stability was not sufficient. Thus the activation of an electrode in which this catalyst was used decreased by approx. 30% within only a few weeks.
It is an object of the invention to provide a novel catalyst for oxygen reduction having desirable and unexpected chemical stability towards highly corrosive media.
It is another object of the invention to provide a novel catalyst for oxygen reduction having desirable and unexpected electrocatalytic activity in presence of organic contaminants.
It is another object of the invention to provide novel gas diffusion electrodes with a novel catalyst therein having desirable and unexpected electrocatalytic properties.
It is another object of the invention to provide a novel electrolytic cell containing a gas diffusion electrode of the invention and to provide an improved method of electrolysing hydrochloric acid to chlorine.
These and other objects and advantages of the invention will become obvious from the following detailed description.
A more effective catalyst having the advantages of the chemical stability of rhodium in the presence of hydrochloric acid is rhodium sulphide. Test electrodes in which RhSx. is used as a catalyst displayed the expected stability after switching off the electrolysis, without polarisation, and the required resistance to catalyst losses due to washing out.
It was however surprisingly found that electrodes in which RhSx is used as the catalyst are almost completely non-sensitive to the broad spectrum of organic and inorganic impurities. Whereas Pt-catalysed electrodes underwent an increase in the operational voltage of up to 260 mV within ten days, even when purified hydrochloric acid was used, and RhOx-catalysed electrodes also underwent an increase of 100 mV under similar conditions, tests using RhSx-catalysed electrodes and purified hydrochloric acid revealed only a slight increase of about 20 mV compared with cells operated with chemically pure hydrochloric acid and only an increase of about 40 mV compared with the value obtained using purified hydrochloric acid even when completely non-purified hydrochloric acid was used. This increase proved to be reversible when purified acid was once again subsequently used. The difference in the operation of the cell when purified technical-grade hydrochloric acid was used as opposed to chemically pure hydrochloric acid has also been demonstrated in additional tests to be between a non-detectable increase in voltage and a maximum increase of 30 mV in the operating voltage of a cell operated under typical electrolysis conditions (current density: 5 kA/m2, operating temperature: 70xc2x0 C., 13-14% HCl).
It is thus by all means advantageous for the technical-grade hydrochloric acid to be pre-purified via an activated carbon line and possibly in addition via an adsorber resin bed, in order to avoid even small increases in the operating voltage. Purification is at any case recommendable, in order to avoid the further reaction of mono- and dichlorobenzene at the anode to form hexachlorobenzene, since the latter is deposited as a solid in the electrolysis unit and the hydrochloric acid loops and can lead to problems especially in valves and pumps after long periods of operation.
An additional finding is noteworthy: oxygen depolarised cathodes of the flow-through type in which the carbon fabric was directly catalysed and which have an open structure, were able to be operated continuously at up to 5 kA/m2 not only with pure oxygen but also with air or depleted oxygen and using organically contaminated hydrochloric acid. The other type used, in which the catalyst is applied to the carbon fabric in a form embedded in electrically conductive carbon dust (the single-sided type) already reached its limits at a content of nitrogen in the oxygen of approx. 30%: The operating voltage was 300 to 350 mV higher and thus already on the borderline of effective operation.
The novel electrochemical catalyst of the invention is comprised of rhodium sulphide, which may be either supported on a conductive inert carrier or unsupported. This catalyst does not require any activation step prior to its use, and surprisingly retains all of its electrocatalytic activity towards oxygen reduction in presence of chloride ions and organic molecules. Moreover, the catalyst is surprisingly not dissolved by the complexing action of aqueous hydrochloric acid/chlorine mixtures, thereby requiring no particular precautions during shut-downs when used in hydrochloric acid electrolysers. The catalyst is preferably coated on at least one side of a web, and may be used alone, with a binder, blended with a conductive support and a binder, or supported on a conductive support and combined with a binder. The binder may be hydrophobic or hydrophilic, and the mixture can be coated on one or both sides of the web. The web can be woven or non-woven or made of carbon cloth, carbon paper, or any conductive metal mesh.
Examples of high surface area supports include graphite, various forms of carbon and other finely divided supports but carbon black is preferred.
Such catalyst coated webs can be employed as gas diffusion cathodes exhibiting cell voltages, current densities and a lifetime that could not be previously obtained under normal operating conditions, especially when used in highly aggressive environments and with low purity reactants, such as the case of electrolysis of by-product hydrochloric acid.
The catalyst may be easily prepared upon sparging hydrogen sulphide gas in an aqueous solution of a water soluble rhodium salt. Nitrogen gas may be used as a carrier for hydrogen sulphide, and a pure nitrogen flow may advantageously be used to purge excess hydrogen sulphide upon completion of the reaction. The resulting solids are recovered by filtration, washing and drying to constant weight at 125xc2x0 C., for example. The rhodium sulphide obtained in this way is unsupported (unsupported catalyst). However, when the aqueous solution of the water soluble rhodium salt further contains a suspension of a suitable conductive support, then the rhodium sulphide is preferentially deposited as tiny particles on the surface of the conductive particles (supported catalyst). The resulting hydrated form of rhodium sulphide must be heated in an inert atmosphere at 550 to 650xc2x0 C., and preferably above 600xc2x0 C. to form an anhydrous form of rhodium sulphide catalyst. The heating may be for several hours depending on the size of the batch, and the choice of the temperature is crucial for the formation of a sufficiently stable catalyst.
If the temperature is too low such as 300xc2x0 C., the resulting crystallites are not well-defined and the catalyst stability is not sufficient. If the temperature is too high, i.e., 725xc2x0 C., the unsupported catalyst has excellent acid stability but is not electrically conductive enough.